An innovation of two established methods for monitoring water colour and clarity: Participatory science using the mini-and midi-Secchi disks

Robert JW Brewin^1*Xuerong Sun¹Joanne Laura Wood¹Thomas G Brewin²Delaney McBride³B. Troy Frensley³Philip J Bresnahan³Stefan Simis⁴Norbert Schmidt⁵Grinson George⁶Ancy C Stoy⁶Nandini Menon N⁷Jasmin Chekidhenkuzhiyil⁷Anas Abdulaziz⁸Shubha Sathyendranath⁴Lilian A Krug⁹Nabanita Das¹⁰Subrata Sarker¹¹Natacha Pasche¹²Laurence Glass-Haller¹²Yasmin Ghadyani¹³Daniel Odermatt^13,14Alexandre Camus¹⁵Suzanne Mader¹⁶Adrien Bonny¹⁶Alexis Pochelon¹⁶Akinnigbagbe Akintoye Edward¹⁷Mauricio Almeida Noernberg¹⁸Edem Mahu¹⁹Ousmane DIANKHA²⁰Adriana Gonzalez Silvera²¹Waqar Ahmed²²Samina Kidwai²³Rodrigo Hernandez-Moresino²⁴Ahmed Hussein Mohamed²⁵Wilfried Essowè Panassa²⁶Noir Primadona Purba²⁷Stella Patricia Betancur Turizo²⁸Lamona Irmudyawati Bernawis²⁹Houssem Smeti^30,31Happiness Anold Moshi³²

• ¹University of Exeter, Exeter, United Kingdom
• ²Chatham and Clarendon Grammar School, Ramsgate, United Kingdom
• ³University of North Carolina Wilmington, Wilmington, North Carolina, United States
• ⁴Plymouth Marine Laboratory, Plymouth, England, United Kingdom
• ⁵DDQ Pocket Science, Heerlen, Netherlands
• ⁶Indian Council of Agricultural Research (ICAR)-Central Marine Fisheries Research Institute, Cochin, India
• ⁷Nansen Environmental Research Centre India, Kerala University of Fisheries and Ocean Studies (KUFOS) Amenity Centre, Cochin, India
• ⁸Council of Scientific and Industrial Research (CSIR)-National Institute of Oceanography, Regional Centre Cochin, Cochin, India
• ⁹Partnership for Observation of the Global Ocean, Centro de Ciências do Mar do Algarve (CCMAR), University of Algarve, Faro, Portugal
• ¹⁰Partnership for Observation of the Global Oceans, Plymouth, England, United Kingdom
• ¹¹Department of Oceanography, Shahjalal University of Science and Technology, Sylhet, Sylhet, Bangladesh
• ¹²Limnology Center, École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland
• ¹³Swiss Federal Institute of Aquatic Science and Technology, Dübendorf, Switzerland
• ¹⁴University of Zurich, Zürich, Zürich, Switzerland
• ¹⁵Université de Lausanne, Lausanne, Vaud, Switzerland
• ¹⁶Association pour la Sauvegarde du Léman, Geneva, Switzerland
• ¹⁷Nigerian Institute for Oceanography and Marine Research, Lagos, Nigeria
• ¹⁸Center for Marine Studies, Federal University of Paraná, Pontal do Paraná, Brazil
• ¹⁹Department of Marine and Fisheries Sciences, University of Ghana, Legon, Ghana
• ²⁰Université Iba Der THIAM, Département Hydro Sciences et Environnement, UFR Sciences et Technologies, Thiès, Senegal
• ²¹Universidad Autónoma de Baja California, Ensenada, Ensenada, Baja California, Mexico
• ²²National Institute of Oceanography, Karachi, Pakistan
• ²³School of Maritime and Applied Sciences, Bahria University Karachi Campus, Karachi, Pakistan
• ²⁴CONICET Centro de Estudios de Sistemas Marinos (CESIMAR), Puerto Madryn, Chubut, Argentina
• ²⁵School of Applied Sciences, Edinburgh Napier University, Edinburgh, United Kingdom
• ²⁶Université de Kara, Kara, Togo
• ²⁷Padjadjaran University, Bandung, West Java, Indonesia
• ²⁸Centro de Investigaciones Oceanográficas e Hidrográficas (CIOH), Cartagena, Colombia
• ²⁹Research Group of Environmental and Applied Oceanography, Faculty of Earth Science and Technology, Institut Teknologi Bandung, Bandung, Indonesia
• ³⁰Sustainable Seas Scientific Studies, Dawnview Crescent, Victoria, Canada
• ³¹University of Monastir, Monastir, Monastir, Tunisia
• ³²University of Dar es Salaam, School of Aquatic Sciences and Fisheries Technology, Dar es Salaam, Tanzania

Measurements of water colour and clarity are used to track the environmental status of lakes, estuaries and oceans. The oldest standardised methods for measuring water clarity and colour are the Secchi disk and Forel-Ule colour scale. Both techniques were developed in the 19th century and require use of the human eye. Despite the advent of optoelectronic-based sensing, these visual methods are still used today, owing to their sensitivity, affordability, simplicity and long history of use. Recently, a hand-held device was developed for measuring the Secchi depth and Forel-Ule colour (presented in two formats, named the mini-and midi-Secchi disk). Designed to be small, light and convenient-to-use, it is well suited for participatory science projects that involve monitoring water colour and clarity. To date, over 900 mini-and midi-Secchi disks have been distributed to citizens and scientists, primarily through six projects, with data mostly transferred via mobile phone applications to data servers and dashboards. In this paper, we describe the methods used in the projects and show some characteristics of the datasets collected so far. We showcase how the device can be used for scientific applications, such as verifying satellite data products, gaining new scientific insights, and supporting public engagement and education. Finally, we provide suggestions for methodological improvements and future developments.